It's a bit small - if that's OK just run it. If you want some power back:1. use an Indian (internally vented) 1-1/16" or 1-1/8" venturi, or2. cut a notch in the leading edge of your M-51 venturi: , or3. try to modify your M-88 body to get correction air from outside (doesn't sound easy).

I have a gut feeling that back in the day H-D was more concerned about mixture control, and that there were actually differences between, for example, an M-41 (45") and M-51 (80") than just the stamp. One possible is that the drilled external vent allowed air correction to be done based on model, but still keep the venturis interchangable.Then the accountants got it - and they decided "one size fits all" saves money, and who cares? The OHV is selling well.